One flight attendant told Business Insider: "Flight attendants will not drink hot water on the plane. They will not drink plain coffee, and they will not drink plain tea."

And that's because not even the cabin crew know the cleaning schedule for the tanks where the water is kept. In fact, tests in 2013 found a high percentage of airplanes had bacteria in the water served on board.

So next time you spot a member of cabin crew purchasing a cup of coffee before boarding a flight, you know why.

It goes without saying that it's important to remain hydrated while flying (the Aerospace Medical Association recommends consuming eight ounces for every hour you're in the air), so make sure you stock up on water in duty free before you board.